En un mundo inundado, un niño rescata a una niña robótica del mar. Agradecida, promete ser su «pierna» hasta cumplir la orden de su antiguo amo, lo que la llevará a una inolvidable aventura veraniega.

This is a tale about humans and vampires. Taishou Romantism: the blooming era. Ladies and gentlemen chat in cafes about the avant-garde and other European trends. But this tranquil lifestyle is destroyed, when vampires start to randomly attack the living. The government of Japan has known about their existence for some time now, but now that these creatures start their hunt against the humanity, the Japanese government creates "Code Zero," a corps that is created to get rid of the bloodthirsty undead.